Cryopumps currently available, whether cooled by open or closed cryogenic cycles, generally follow the same design concept. A low temperature surface, usually operating in the range of 4 to 25 K, is the primary pumping surface. This surface is surrounded by a higher temperature surface usually operated in the temperature range of 70 to 130 K, which provides radiation shielding to the lower temperature surface. In addition, this higher temperature surface serves as a pumping site for higher boiling point gases such as water vapor. The radiation shielding generally comprises a housing which is closed except at a frontal array positioned between the primary pumping surface and the chamber to be evacuated. In operation, high boiling point gases such as water vapor are condensed on the frontal array. Lower boiling point gases pass through that array and into the volume within the radiation shielding and condense on the primary pumping surface. A surface coated with an adsorbent such as charcoal or molecular sieve operating at or below the temperature of the primary pumping surface may also be provided in this volume to remove the very low boiling point gases. With the gases thus condensed and or adsorbed onto the pumping surfaces, only a vacuum remains in the work chamber.
In systems cooled by closed cycle coolers, the cooler is typically a two stage refrigerator having a cold finger which extends through the rear of the radiation shielding. The cold end of the second coldest stage of the cryocooler is at the tip of the cold finger. The primary pumping surface or cryopanel which is connected to a heat sink at the coldest end of the second stage of the coldfinger may be a plain metal surface or an array of metal surfaces arranged around and connected to the second stage heat sink. The primary pumping surface contains the low temperature adsorbent. A radiation shield which is connected to a heat station at the coldest end of the first stage of the coldfinger surrounds the primary cryopumping panel in such a way as to protect it from radiant heat. The radiation shield must be sufficiently spaced therefrom to permit substantially unobstructed flow of low boiling temperature gas from the vacuum chamber to the primary pumping surface. The frontal radiation shield is cooled by the first stage heat sink through the side shield. Typically, the temperature differential across that long thermal path from the frontal array to the first stage heat sink is between 30 and 50 K. Thus, in order to hold the frontal array at a temperature sufficiently low to condense out water vapor, typically less than 130 K, the first stage must operate at between 80 and 100 K.
The heat load which can be accepted by a cryocooler is strongly temperature dependent. At high operating temperatures conventional cryocoolers can accept higher heat loads. Thus, a reduction in the temperature differential between the frontal array and the first stage heat sink will allow an increase in the operating temperature of the first stage heat sink. This will allow the cryocooler to accept a higher heat load while maintaining the frontal array at an acceptable operating temperature. To accomplish this reduction in temperature differential, conventional cryopump designs utilize high conductivity materials such as copper in the radiation shields. The gradient can be further reduced by increasing the cross sectional area of the radiation shielding to thus increase the thermal conductance of that shielding. This increased mass of the shielding adds both weight and cost to the product and disadvantageously increases the cool down time and regeneration time of the cryopump.
An object of this invention is to provide a cryopump which minimizes the temperature differential between a cryopanel and associated heat sink without substantially increasing the mass of the system while at the same time allowing the cryocooler to operate at a higher loading level (higher temperature).